The Role of Resistivity Logging to Determine Injectivity Height on Injection Fall-Off and Diagnostic Fracture Injection Testing in Coal Seams

Recently, a program of injection fall-off (IFOT) and diagnostic fracture injection (DFIT) tests was performed in coal seams in an exploration well in the Bowen Basin. Prior to testing, a minerals log suite was acquired to determine intervals for testing and to aid in pre-design test parameters based on logs and offset well data. Post testing, an additional multi array resistivity log, was used to evaluate the well and interburden properties. As the IFOT utilised fresh water in the more saline coals, this produced interesting results whereby the actual post-test invasion profiles clearly indicated the permeability-height product (kh) tested during each injection. The logs showed a differential in the invaded coal between the pre- and post-testing resistivity and an increase in the spontaneous potential (SP) post-test based on relative invasion. We will show examples of the types of responses observed in this well and discuss the reasons for potential non-responses. We will outline the process, the environment, and required logs to utilise this technology and the additional logs that could be deployed to minimise uncertainties further when applying this methodology. Alternatively, non-radioactive and radioactive tracers have been employed to evaluate test heights; however, both methods have problems with handling/disposal, density segregation, and miscible displacement or dilutions to levels beyond their respective post-test investigation tool radii. In conclusion, this qualitative methodology provides a low-cost means of reducing the uncertainty in the determination of the actual kh for an IFOT or DFIT.